1. The Field of the Invention
This invention relates in general to launching and collecting light for inelastic optical spectroscopy. Specifically, the invention shows how to preserve the spectral purity of the launched light with all reflective optics while simultaneously collecting light from a sample with a wide field of view, re-using the reflective optic nearest the sample and a filter which is used both to reflect the launched light and pass the in-elastically scattered light.
2. Background and Relevant Art
In performing inelastic optical spectroscopy, a sample is irradiated with optical radiation in one wavelength band and emits radiation in a second wavelength band. If the excitation radiation passes through transmitting optical elements prior to impinging on the sample, it is possible for inelastic radiation to be produced by the optics, some of which radiation then being scattered by the sample and interfering with the scattering spectrum of the sample. In consequence, a reflective optic is sometimes employed to both focus the incident radiation on the sample and collect the scattered light from the sample. An example of such an apparatus is disclosed by Messerschmidt in U.S. Pat. No. 5,225,678 where symmetrical pairs of identical parabolic mirrors are used for collection and refocusing. A well known property of such a symmetrical system is that some aberration cancellation can thereby be achieved. In a continuation-in-part application that issued as U.S. Pat. No. 5,311,021, Messerschmidt further discloses an apparatus particular to Raman spectroscopy which is one form of inelastic spectroscopy. Referring to FIG. 14 of U.S. Pat. No. 5,311,021, the input beam is brought to the sample via a hole in a mirror 16, mirror 16 also reflecting scattered radiation from the sample to the detector. A mirror with a hole will exhibit losses with respect to the reflected scattered radiation, which may be important when the scattered signal is weak. Another problem is that laser light may be scattered from the aperture in the mirror, or from optics on the side of the mirror opposite the excitation source, some of which scattered radiation may be incident on the spectroscopic apparatus. In inelastic spectroscopy, rejection of the excitation wavelength is often critical because the scattered radiation may be many orders of magnitude weaker than the excitation radiation, hence it is helpful to avoid the creation of sources of unwanted excitation radiation that may be incident on the spectrometer.
It is possible to perform the function of separating excitation and emission spectra, using an interference reflector, as is taught by Wheatley et al., in U.S. Pat. No. 7,497,608 B2. In Wheatley, the reflector transmits the emission wavelengths while reflecting the excitation wavelengths. The apparatus of Wheatley was intended for illumination only, and in particular, it is not suitable for both illumination of a sample and collection of the scattered radiation from the same side of the sample. The apparatus of Wheatley would require optical access to a second side of the sample which is often impractical.